The invention relates to monitoring a road surface condition on which a vehicle is travelling, and more particularly to a method and apparatus for continuously determining a slip condition of a vehicle which is directly related to the coefficient of friction of the road surface.
The invention also relates to a method and apparatus for applying a controlled vertical (i.e., normal) force to a test wheel of a friction monitor and to a friction monitor using such a vertical force monitor in combination with an electromagnetic torque motor for producing slippage of a test wheel.
The invention further relates to a device for monitoring a surface (e.g., road, runway, rail line etc.) condition on which a monitor (e.g., vehicle, airplane, train etc.) is travelling, and more particularly to a method and apparatus for determining a slip condition of a vehicle which is directly related to the coefficient of friction of the road surface while utilizing an electromagnetically controlled vertical force applied to the test wheel.
The availability of quantitative information representative of the coeficient of friction of the road surface is very beneficial to drivers of moving vehicles, including planes. An exact knowledge of how slippery the surface is, continuously provided, gives the driver significant advantages in determining safe speeds, distances from other automobiles, acceleration and braking patterns etc. so as to permit a safer operation of the vehicle, plane or apparatus being driven.
Previous devices for monitoring the coefficient of friction had significant disadvantages such as, for example: a failure to provide continuous road friction data over long distances; the use of complex electro-mechanical-hydraulic mechanisms with unfavorable wear characteristics; limited, specificity, variability and responsiveness of test wheel(s) braking force; the use of only indirectly measured values of the force (torque) required to produce slippage; employment of equipment which was unsuited for continuous use close to the ground and in inclement conditions; the use of a cumbersome test wheel suspension and carriage; employment of equipment which produced adverse affect on normal drive operation; and the use of relatively complex designs which produces an expensive apparatus of only limited use.
When measuring the coefficient of friction, various means of applying vertical force have been employed, such as: springs, coils, rubber, compressed air bags, pistons containing air, gases, oil. The foregoing vertical force applying devices have significant limitations. The vertical force tends to increase as tensioning means are compressed and decrease when decompressed. The vertical force is difficult to measure, especially if continuous or frequent measurements are desirable. Further, one is generally not able to standardize the force from one system to another. The vertical force tends to change with use/wear due to deterioration of compressed materials/mechanisms, and to vary with change in temperature and/or moisture.
Several patents are directed to devices which do not employ a separate test wheel but rather employ one of the usual drive or driven wheels of the vehicle. Among these patents are U.S. Pat. No. 4,882,693 to Yopp and U.S. Pat. No. 4,545,240 to Leiber. These devices necessarily adversely effect the driving characteristics of the vehicle.
The Yopp patent measures forces acting on a steering column, such as the steering angle combined with other data.
Other patents teach the use of a test wheel or probe which is lowered into contact with the road surface to perform the desired measurement. Among these patents are U.S. Pat. Nos. 4,098,111 and 4,212,063 to Hardmark; U.S. Pat. No. 4,958,512 to Johnson; U.S. Pat. No. 3,893,330 to Shute; U.S. Pat. No. 4,315,426 to Brandon; U.S. Pat. No. 4,662,211 to Strong; and U.S. Pat. No. 4,909,073 to Takahashi. These patents employ complex devices which are not adapted for continuous operation and wear and which are bulky and complicated in their operation.
One object of an aspect of the invention is to continuously monitor road surface friction characteristics under a wide variety of road conditions and vehicle velocities.
Another object of the invention is to provide a slippage indication by simulating the actual physical circumstances of slippage of a braked or accelerated wheel of a vehicle.
A further object of the invention is to monitor road surface friction without interfering with the movement of the vehicle wheels or braking system such as would affect the directional path or speed of the vehicle, and in particular, not to affect the freedom of the drive wheels or other wheels of the vehicle or interfere with the braking system, thereby potentially throwing the car off its directional path or causing acceleration or deceleration of the vehicle.
Yet another object of an aspect of the invention is to provide a lightweight, comparatively small monitoring device of suitable weight and size for mounting on an automobile or plane that operates independently of any other system in a moving vehicle, that is comparatively inexpensive to manufacture, and that does not require much maintenance.
Another object of an aspect of the invention is to provide a simple, easily retractable, and easily detachable slip detection device which does not constitute a significant obstacle to the movement of the vehicle.
Another object of an aspect of the invention is to continuously provide near instantaneous, widely variable, highly specific, easily directly measurable, braking or accelerative force; which does not vary, fade, or fail, at any vehicular speed.
Another object of an aspect of the invention is to measure friction conditions without utilizing cumbersome direct mechanical friction devices with their attendant mechanical wear.
Yet another object is to provide a slip detection device suitable for after market mounting on existing vehicles.
It is a further object of an aspect of the invention to provide a compact suspension mechanism for applying and maintaining a controlled, accurately measurable vertical force to the test wheel of a friction monitor, which vertical force is unaffected by variations in temperature, moisture, or atmospheric pressure; remains constant as the test wheel moves up and down with reference to its point of support due to variations in the contour of the measured surfaces; and counters the tendency of the test wheel to move upwards and lose contact with the measured surface. The vertical force may be varied in a controlled manner during use.
The emphasis with the present invention, in accordance with an aspect thereof, is on the continuous determination of road surface friction characteristics for automobile drivers or pilots, so as to facilitate decision-making with regard to safe travelling speed, braking distance, cornering speed, acceleration speed and the like in various road conditions; wet (hydroplaning), snow, slush, ice, oily surface, etc. Data on varying road coefficient of friction characteristics is continuously presented to the driver as a visual and/or auditory signal.
An embodiment of the present invention provides a means for continuously evaluating the coefficient of friction of the road by providing a relative quantification of the coefficient of friction. The device is comparatively compact and relays to the driver a visual and/or auditory signal indicating relative slipperiness of the road. For example, a suitably placed, non-distracting flashing light could be used; the flashing light can be designed to flash more frequently and more brightly as the slipperiness of the road increases. An audio signal can also be used to inform the driver about road conditions: again, the frequency and intensity of the audio signal can be increased in relation to the slipperiness of the road.
An embodiment of the present invention mimics the strategy commonly used by drivers to evaluate road conditions: pressing on the brake to determine how hard they have to press (decelerative force) in order to produce a limited skid, alternatively, accelerating quickly, to observe how much accelerative force is required to make the drive wheels skid. This device repeatedly and automatically carries out this function. Drivers know that the harder they have to press on their brakes or accelerator in order to produce wheel slippage, the less skiddy the road is. This concept is the basis of the present inventionxe2x80x94the more braking resistance or accelerative force required to produce skidding, the greater the coefficient of friction.
The device, through the use of variable resistance or accelerative force, creates slippage of a small test wheel and then detects the earliest sign of skidding of this wheel and measures the amount of force required to produce slippage of the test wheel. In accordance with another aspect of the invention, a processor can combine signals from the circuit regulating the vertical force torque device, with signals from any other circuits, in particular, the circuit to control a torque motor used to produce slippage when measuring a surface (e.g., road, runway, rail line) condition.
An embodiment of the invention utilizes an electromagnetic force field, as distinct from direct mechanical means (springs, air pistons, rubber . . . ) to provide the requisite vertical force acting on the surface of the test wheel.
Information regarding this force is then relayed to the driver. An elementary embodiment of the invention has an open loop control system and a fixed electromagnetic field resistance to the rotation of a test wheel. A warning signal is provided to the driver when the test wheel skids indicating that the road surface has reached a potentially hazardous threshold.
Another embodiment of the invention utilizes an open-loop control system and a variable braking resistance. A series of predetermined resistance levels are applied, and the test wheel slip condition is conveyed to the driver at each state, providing a more refined indication of road slipperiness.
Another embodiment of the invention utilizes a closed feedback loop control system with variable braking resistance or accelerative force. The test wheel is first allowed to rotate freely as the vehicle travels. A timer initiates the test process, and variable resistance (or accelerative force) is applied to the test wheel using, for example, an electric motor, designed to produce torque in the same or opposite direction (or same direction in the case of acceleration) to the rotation of the test wheel. The braking resistance or accelerative force is increased until the test wheel slips. A sensor determines the rotational speed of the test wheel and, from this test wheel rotational speed in any given period of time, the amount of slip of the test wheel is determined. The amount of braking resistance or accelerative force required to generate wheel slip is measured, and this information is conveyed to the driver by various display methods. The variable braking or accelerative force is then decreased and the process is repeated.
Another embodiment of the invention employs a second or reference wheel. This reference wheel has no brake and serves solely as a reference tachometer to facilitate determination of the percentage slip of the braked or accelerated test wheel. Optionally this reference wheel can power a generator, which, generator can provide E.M.F. to the electric motor and/or battery.
A test wheel is used, as opposed to actual vehicle wheels, to ensure that the repeated road surface monitoring does not affect normal vehicle operation. The test wheel is designed to be easily retractable and instantly detachable to minimize the effects of roadway obstacles. The desire for simplicity and low cost leads to a method that uses increasing braking resistance or accelerative force to induce slip instead of a geared test wheel driven at a predetermined slip, and that monitors road surface conditions to produce a relative quantification of the actual coefficient of friction. Similarly, variable resistance to rotation of the test wheel is provided by an electric motor, instead of cumbersome mechanical brakes.
Another embodiment of the invention is directed to a method of and apparatus for applying and maintaining a vertical force, which may be variable if desired, to a test wheel of a friction monitor utilizing an electric torque device, such as a motor, under control of an electronic processor such as a digital computer or the like.